Mutualisms are defined as
‘mutually beneficial
interactions among different species’, they have a tremendous
importance in
ecology, evolution, and economics. There are some 1 million insect
species
described. These often are known for their negative interactions with
one or
some of the approximately 300.000 species of higher plants (herbivory),
but on
the other hand many plant-animal interactions in fact are beneficial
for both
or all partners involved. Presumably all higher plants moreover are
involved in interactions with bacteria and
fungi, many of
which are mutualistic, too. Such mutualisms are based on the exchange
of
valuable resources and therefore are endangered of being exploited by
other
species, which then act as parasites of the mutualism. Specific
“filters” allowing
protection of mutualisms from exploitation thus are important for their
evolutionary stabilisation. This remains true both for very generalised
mutualisms
(such as pollination and seed dispersal by animals) and highly
specialised ones
(as, e.g., the interaction among bull’s horn Acacia
and Pseudomyrrmex-ants,
one of our study objects).

Indirect
defence and plant-plant communication (Martin Heil, Daniel Ballhorn,
Jan Preuß).
Many plants respond to herbivore damage by the release of volatile
organic
compounds (VOCs) or the secretion of extrafloral nectar (EFN). These
traits
then attract predators or parasitoids (mainly ants and wasps) that
attack the
herbivores and thereby indirectly defend the plant. We used wild Lima
bean (Phaseolus lunatus) growing in México to
demonstrate that these “indirect defences” indeed benefit plants under
natural
conditions. Moreover, plants can “communicate” among each other: Lima
bean
plants (or leaves) that were exposed to VOCs increased their EFN
secretion
immediately, i.e. they used the information that neighbouring plants
are
already attacked to adjust their defensive phenotype accordingly.
Future
studies will include the Lima bean’s direct defence (cyanogenesis) to
investigate how all these different defence mechanisms interact in
plants from
different natural populations. In order to obtain a first impression of
the
evolutionary history of these phenomena we are now also working on
bracken fern
(Pteridium aquilinum), a
non-flowering plant that also produces EFN and VOCs. No beneficial
(i.e.
defensive) effects of VOCs and EFN could be demonstrated so far for
bracken
fern, an observation pointing to putative other (plant-internal?)
functions of
these traits. This system thus will enhance our understanding of the
evolution
of these widespread defensive plant traits.

Pseudomyrmex-Acacia Symbioses. Ant-plant mutualisms
represent a particular form of
indirect defence of plants, since ants provide the defensive effect for
plants
against herbivorous animals. Such mutualisms occur at different degrees
of
specificity, ranging from facultative to highly obligate. They
therefore form
an ideal model for comparative studies. In the genus Acacia,
plants produce extrafloral nectar (EFN) in order to attract
or nourish ants. In the case of obligate mutualisms, this extrafloral
nectar is
secreted constitutively, whereas, in contrast, Acacia
species being only facultatively inhabited by ants exhibit
an inducible nectar secretion.

Invertase activity in Pseudomyrmex ants (Stefanie Kautz). We recently showed that EFN
of myrmecophytes
contains no sucrose, which makes it very unattractive for generalistic
ants.
Ant species specialised to live on these plants accordingly lost the
enzyme
required to digest sucrose, invertase. This enzyme is highly activite
in the
guts of related, yet unspecialised ants. We are now conducting
phylogenetic
studies using several molecular markers in order to identify the level
at which
invertase has been lost within the ant genus. The structure of the
protein is
completely unknown and therefore will be characterised. Moreover, we
are investigating
further digestive enzymes that might have adapted to the special diet
of
obligate plant-ants.

Bacteria in the ant-Acacia
association (Sascha Eilmus) Just like humans ants contain numerous
bacteria in
their gut, and it is unclear whether these bacteria are responsible for
the
production of the invertase that has been detected in the gut of
generalistic
ants. This leads to the general question as to whether the bacterial
communities to be found in specialised and non-specialised ants differ
from each
other: Did the mutualistic ants probably loose some of their bacterial
symbionts? And: are there also differences in the soil bacteria to be
found in
the root systems of specialised and non-specialise plants? These
questions are
treated with different culturing techniques as well molecular
techniques for
the sequence-based identification of bacteria (PCR/ t-RFLP) in order to
cover non-culturable
bacteria as well.

Plants have
to face multiple enemies. Among these, insect herbivores represent a
prominent
group of plant antagonists. As sessile organisms plants have to defend
themselves against attackers to ensure their survival and reproduction.
But what
means “plant defence”?

In general,
direct and indirect defence mechanisms can be distinguished. Direct
defences include
structural traits, toxins or repellents — and thus interact directly
with the
attacker — in contrast, indirect defences, comprise a further trophic
level: predators
or parasitoids of the herbivores are attracted by the plant. They prey
on the
herbivores and as a consequence reduce herbivore pressure. A well-known
example
for such an indirect defence represents the herbivore-induced emission
of plant
volatiles (volatile organic compounds; VOCs). These substances are
released in
response to leaf damage caused by the herbivores and mean an attractive
olfactory cue to carnivorous insects. Various indirect defence
mechanisms had
been studied intensively over the last years using different plant
species as
model organisms. Among these species Lima bean (Fabaceae: Phaseolus
lunatus L.) represents one of the most important experimental
plants in biochemical ecology.

In
contrast to numerous studies on indirect defences of Lima bean, its
direct defences
had been neglected so far. However, own recent studies demonstrated
high
efficiency of lima beans’ cyanogenesis in anti-herbivore defence.
Cyanogenesis is
considered a direct defense and means the release of toxic hydrogen
cyanide (HCN)
from inactive precursors in response to injury of tissues (for example
by
herbivores). I am fascinated by the molecular and ecological interplay
of
direct and indirect defences. Consequently, I combine laboratory and
field
research in order to assure that phenomena studied under controlled
situations
are of relevance under natural conditions.

Field
research is conducted on natural populations of lima bean in South Mexico (States of
Oaxaca and Veracruz)
in cooperation with Dr. Martin Heil, Departemento de Ingenieria
Genética,
CINVESTAV, Irapuato,
Guanajuato, Mexico.

Humans are not the only
organisms that can defend
themselves against infection and disease. When plants are infected by
pathogens, cell walls are lignified and toxic compounds synthesized
both in the
tissue infected and in other, yet healthy parts. Moreover, enzymes are
produced
that hydrolyse cell walls of bacteria and fungi. This so-called
systemic
acquired resistance (SAR) has a broad-spectrum efficiency against many
types of
different diseases and can be activated artificially. It thus is the
target of
large efforts in research on agronomic plant protection.

We
are interested in answering the question why the plants wait at all for
being
infected, instead of activating their resistance constitutively. Recent studies demonstrated that SAR is
very costly.
Plants
have to allocate important resources to their resistance that
thereafter cannot
be used for other important processes such as flowering or fruit set.
If a
plant invests in resistance under conditions not requiring it, this
plant grows
less efficient than others.

In a current project that is
conducted in cooperation with the Dept. of Bioorganic Chemistry,
Max-Planck-Institute of Chemical Ecology, Jena, Germany (www.ice.mpg.de), we are
investigating the
proteome of the chloroplast of Arabidopsis
plants in order to illustrate and understand re-allocation phenomena
that occur
after resistance elicitation. During the upregulation of SAR, plants
apparently
degradate important proteins that function in leaf photosynthesis in
order to
synthesise pathogenesis-related proteins (PR proteins).

Moreover,
plant-microbe interactions are not restricted to disease. Almost all
plants are
also engaged in mutualistic interactions with some kind of bacteria and
fungi,
just as humans profit from an intact gut microflora. Virtually all
members of
the leguminosae (e.g. pea, bean and lupine) house bacteria of the genus
Rhizobium in specialised structures of
the root; these bacteria provide the plant with nitrogen in the form of
ammonium that is reduced (“fixated”) from atmospheric dinitrogen.
Similarly,
roots of most forest trees are associated with fungi that facilitate
nutrient
uptake. We are currently investigating whether SAR can negatively
affect these
beneficial microorganisms. Ecological costs – costs resulting from a
negative
effect of a resistance on important ecological interactions of the
plant – are
another factor that might affect both, the evolution of SAR and its net
benefit
when it is to be used as a plant protection strategy in agronomy.

Projekt 3
(Ballhorn)
Title: Cyanogenesis
and Endophytic Fungi Communities in Giant Bamboo and Effects
on the Interaction with Bamboo Lemurs

Giant bamboo
(Poaceae: Bambusoideae:
Cathariostachys madagascariensis (A. Camus) S. Dransf.) is
endemic to Madagascar.
This bamboo species is characterized by strong cyanogenesis.Although generally considred an efficient
chemical defence against herbivores giant bamboo can
account for as much as 95% of the diet of the
highly endangered greater bamboo lemur (Prolemur
simus). In contrast to the wide-ranging distribution of C.
madagascariensis,
occurrence of its herbivore is highly restricted. P. simus
is threatened by environmental degradation (slash-and-burn
agriculture, logging, as well as the extensive cutting of bamboo) and
hunting.
However, these factors still unsufficiently explain the restriction to
presumably
small areas. I hypothesize strong variability of cyanogenesis among
populations
of C. madagascariens and, in consequence restriction of P. simus quantitatively depending on
this plant trait.

In
this project quantitative variability of cyanogenic features among
bamboo populations
will be traced on the genetic diversity of bamboo as inferred by an
AFLP
analysis (applied fragment length polymorphism). In addition,
cyanogenic plant
features will be compared to colonization by endophytic fungi of the
same plant
individuals. Endophytic fungi – microfungi that colonize and live
within
healthy plant tissue without inducing symptoms of disease – comprise a
large
but little explored group of plant symbionts with potentially enormous
ecological
impact. To what extent C. madagascariensis is colonized by
endopyhtic
fungi, how these fungi affect the plants nutritive quality and to what
extent
these fungi synthesize bioactive compounds that interact with P.
simus
is completely unknown.

The
comparative analysis of ecological effects of plant cyanogenesis and
production
of secondary bioactive compounds by fungal endophytes under natural
conditions
signifies an approach that is new to biochemical ecology. The project
will
provide a better understanding for the functional ecology of complex
systems. Aim of this project is to investigate
causal effect of bamboo food qualitiy on the restricted distribution of
P.
simus. A better understanding of multi species interactions
is of
great importance for basic sciences as well as for conservation biology
of
endangered species.